de Melo Oliviera, Thompson 1 Spillway Design Victor de Melo Oliviera Luke Thompson CIVE 401 shown below in figure 1. Control structures regulate the flows from the reservoir into the spillway. ensuring that flow will not enter the spillway until the water in the reservoir reaches the designed level. De Melo. both of which can fail completely when overtopped. and moderating flow into the spillway once the design level has been reached. Spillway at the Discharge channels. are defined in the Design of Small Dams as a provision “for storage and detention dams to release surplus water or floodwater that cannot be contained in the allotted storage space…1. Plunge basins. Thompson 2 Spillways. which can be particularly destructive in the case of earthfill and rockfill dams. or pipes. Figure 1. Note that conveyance structures are not always presentin a spillway design. Spillways generally transport excess water that cannot be contained behind a dam to the streambed below it. . discharge channel. Design of Small Dams. and entrance/outlet channels. which oftentimes acquires a high velocity while traveling down a spillway. Spillway Components Spillways generally are made up of four components: a control structure. convey flow that Hoover Dam. weirs. Pg. terminal structure. or any other nearby structures. passes through the control structure down to the streambed below the dam. This action prevents overtopping. Entrance channels convey water from a reservoir to the control structure. at times discharge may fall freely after passing through the control structure. orifices. 339. and deflectors are all examples of terminal structures. flip buckets. Control structures can be sills. will not cause excessive erosion to the toe of the dam. 3 ed. Outlet channels convey flow that has reached the terminal structure to the river channel that resides below the dam. Terminal structures ensure that the flow. also known as waterways. 1987. 1 rd United States Department of Interior-‐Federal Bureau of Reclamation. shown to the left. “For discharges at design head. 353. de Melo Oliviera. Figure 3. because less head is required. Additional crest length created and the spillway will be dealing with by triangular folds. Labyrinth spillway. and culvert. because as stated in Design of Small Dams. ogee. open channel. Thompson 3 Entrance and outlet channels are not necessarily a component of all spillways. conduit. Spillway Types Common types of spillways include: free overflow. pg. it is possible for the spillway to transport flow directly from the reservoir to the river channel. boundary surface and attains near-maximum discharge efficiency. tunnel. shown below. as compared to traditional spillways.Ogee spillways can be advantageous. 2 United States Department of Interior-‐Bureau of Reclamation. Labyrinth spillways are particularly useful when spillway width is fixed. the flow glides Figure 2.”2 Labyrinth spillways contain additional crest length in the spillway width. labyrinth. For an example of a spillway crest. Additional crest length allows the spillways to pass a larger discharge than normal. . An example of a labyrinth spillway can be seen in figure 3. baffled apron drop. Free overflow or straight drop spillways are spillways in which flow falls freely after passing over the crest of the spillway. please refer to figure 2. Ogee spillways are characterized by control structures that consist of ogee shaped crests (crests that are s-shaped when viewed in profile). drop inlet. A spillway with basic over the crest with no interference from the components identified. also known as Chute or Trough spillways convey water by means of an open channel. In drop inlet spillways. shown in figure 4. because the designation is applied without reference to the control structure. are used to lower water from level to level. also known as Shaft or Morning Glory spillways. They are ideally used in situations where there is a relatively small maximum outflow. and into the river channel. De Melo. Conduit and tunnel spillways convey flow by means of a closed conduit or closed channel. allowing kinetic energy to dissipate. water flows over a horizontal inlet. Thompson 4 large discharges. culverts have a profile grade that is Dam. Open channel spillways are often simple to design. down a sloped or vertical shaft. in Washington. because maximum capacity occurs at relatively low heads. Culvert spillways are a subset of closed conduit or spillways where the inlet into the spillway is either vertical. Drop inlet spillways are advantageous in situations where dams are situated in narrow canyons. obstruct the water’s flow. and are aerated to prevent them from acting as siphons between the locations of water transport. and it is recommended in Design of Small Dams that baffled spillway designs that require a slope steeper than the aforementioned be tested at the model scale before design proceeds. (nearly) uniform. Baffled. Baffled spillways generally have slopes that are 2 horizontal units to 1 vertical unit at most. Open channel spillways. It should be noted that channels can be open channel spillways and still possess the characteristics that are necessary for them to be identified as another type of channel. and are frequently used in conjunction with earthfill dams. Baffle piers. in situations are stilling tanks are not wanted. which are protrusions along the spillway. Figure 4. or apron spillways. Baffled spillway at Conconully Additionally. Most conduit and tunnel spillways only have flow occupying up to 75% of their cross-sectional area. or inclined to the upstream or downstream. . This profile is advantageous because it can approximate the shape of the jet that travels along its surface. velocity-of-approach head) (L) . 1 where Q = discharge (L3/T) C = variable discharge coefficient g = gravitational acceleration (L/T2) L = effective length of crest (L) H = head being considered on crest. Shown below in figure 5 is a schematic of flow traveling over an ogee spillway. (including ha. taken from Dr. Schematic of flow and related quantities as they pertain to an Ogee spillway. Figure 5. de Melo Oliviera. Pierre Julien’s 11-11-14 handout. consequently making it easier to obtain ideal discharges. entitled CIVE 401-Notes on Spillways. Ogee spillways are spillways that generally have an S-shaped profile. In the case of an Ogee spillway. Thompson 5 Design of Ogee Spillways As mentioned earlier. discharge is given by the following equation: ! ? = ? 2???! Eq. L. 2 where: Q. Ogee crest coefficient vs. Julien’s notes) The ogee crest coefficient can be determined using figure 6. which follow below. ratio of height of spillway to design head. & H are defined as in equation 1a Co = ogee crest coefficient (also called CD in Dr. figures 7 and 8. shown below. leading to the following equation: ! ? = ?! ??! Eq. Thompson 6 The quantity ha accounts for friction and other losses in the channel approaching the crest of the Ogee spillway. and the spillway’s upstream face is vertical. De Melo. if the design head and the actual head acting at the crest are the same. At this point in time. Figure 6. are charts that are able to address the aforementioned issues. allowing the use of the Ogee coefficient. Co. the ogee crest coefficient is not particularly versatile. Co can replace the C in equation 1. However. and the ogee spillway under consideration has a vertical face upstream. However. then the ogee crest coefficient. because it only applies to the situation where design head and head acting on the crest are equal in magnitude. . 3 where: L = effective length of crest (L) L’ = net length of crest (L) N = number of piers Kp = pier contraction coefficient Ka = abutment contraction coefficient H = head on crest (L) Pier contraction coefficients.0 and 0. effective crest length can be computed using the following equation: ? = ?! − 2 ??! + ?! ? Eq. ratio of spillway to Design Head. varies between 0. required for the discharge calculation.0 and 0. Ratio of Actual Head inclined and vertical faces vs. including shape and location of the pier’s nose. Average Ka. the abutment contraction coefficient.2. Kp are functions of several factors. also depending on the shape of the abutments. height to design head.02 depending on the shape a pier’s nose. de Melo Oliviera. Ratio of discharge coefficients for to Ogee Crest Coefficient vs. . in which case. Average Kp’s vary between 0. Ratio of Variable Coefficient of Discharge Figure 8. the actual length of the crest is not the effective length. L. In such a situation. thickness of the pier design head of the spillway. the flow over the crest of the spillway could be contracted. and the velocity of the approaching flow. It is possible that due to the placement of the piers and abutments of the dam. Thompson 7 Figure 7. 19 Nov.leanhtuan. Pg. Figures Figure 1. 1 Figure 6. et al." Bureau of Reclamation: Lower Colorado Region .d. 356 Figure 4. Pg. 3rd ed. Pierre Julien (11-11-14). Pg. Pierre. "Spillways. 1987. United States Department of Interior. et al. Tuan. chapter 17 of the Hydraulic Design Handbook explores the design process of the various kinds of spillways in greater detail as well. Pg. The aforementioned chapter can be found at the following link: http://www. Pg. United States Department of Interior-Bureau of Reclamation. 3rd Edition. LAP LAMBERT Academic Publishing. Pg. Open Channel Hydraulics for Engineers. This text is easily found in PDF form on the internet. et al.org/uploads/Search%20engineering/Spillway%20Design. 1987. Pg. 2014. 371 Figure 8: United States Department of Interior. Pierre Julien (11-11-14). Also found at: http://www.keu92.pdf. n. 2011. Le Anh. De Melo. it is recommended that chapter 9 of the third edition Design of Small Dams be consulted.Hoover Dam: Spillways. . United States Department of Interior-Bureau of Reclamation. Figure 2.com/pdf/OCHC%205. Additionally. CIVE 401-Notes on Spillways by Prof. Web. CIVE 401-Notes on Spillways by Prof. 90. United States Department of Interior-Bureau of Reclamation. and covers much that is beyond the scope of this paper. Thompson 8 References Julien. United States Department of Interior. et al. Design of Small Dams. 370 Figure 7: United States Department of Interior.pdf Figure 3. Julien. Design of Small Dams. 360 Figure 5. Pierre. 371 Further Reading For further information concerning the design of spillways.